1. Field of the Invention
The present invention relates to a memory unit. More particularly, the present invention relates to a non-volatile memory unit.
2. Description of Related Art
Random access memory can be classified into dynamic random access memory (DRAM) and static random access memory (SRAM). SRAM is advantageous in high speed operation and low power consumption. Moreover, compared with DRAM, SRAM does not need to be re-charged periodically, and is simpler in design and fabrication. Thus, SRAM is widely employed in information electronic products.
SRAM is a volatile memory, which stores data according to the conductive state of transistors inside a memory cell. Therefore, when the power supply for the memory is shut down, the data stored in the SRAM will be lost completely.
In another aspect, electrically erasable programmable read only memory (EEPROM) has become a non-volatile memory element widely used in PCs and electronic equipments due to the advantages of storing, reading, erasing data repeatedly, and keeping the stored data even if the power shut down.
However, with the continuous progress of science and technology, IC technique is developing rapidly, and information electronic products are varied, such as computers, mobile phones, digital cameras, and personal digital assistants (PDAs). Accordingly, more data are required to be processed and stored by the information electronic products, and meanwhile the characteristics of light, thin, short, small, and portability of information electronic products must be taken into consideration. Therefore, in order to prevent data stored in an SRAM from getting lost when the power supply is shut down, it is required to provide a semiconductor device having the advantage of high speed operation of an SRAM and the function for storing data of a flash memory.